Transistorized grid pulsing circuit for x-ray tubes and other purposes

ABSTRACT

A light-emitting diode is pulsed at a preselected rate. Emitted light is conducted through a glass image conduit to a phototransistor which operates a solid state control switch at the same rate. The solid state switch controls conduction of a series connected string of transistors whose collectors swing from essentially zero voltage when the transistors are conducting to a high voltage when they are not conducting. The circuit features means for turning all of the transistors off in such manner that no transistor is subjected to over-voltage. The voltage pulses developed on the collector of the last transistor in the series may, for example, be applied to the grid of an xray tube to control its conduction intervals during high rate cinefluorography with an x-ray image converter. A signal that depends on image brightness is used to modulate the width of the pulses and, hence, the output of the x-ray tube so that each film frame has uniform exposure.

United States Patent I [1 1 Lambert July 31, 1973 TRANSISTORIZED GRIDPULSING CIRCUIT FOR X-RAY TUBES AND OTHER PURPOSES {75] Inventor: ThomasW. Lambert, Hales Corners,

Wis.

[73] Assignee: General Electric Company,

Milwaukee, Wis.

[22] Filed: Oct. 16, 1970 [21] Appl. No.: 81,504

Related US. Application Data [62] Division of Ser. No. 801,376, Feb. 24,1969, Pat." No.

[52] US. Cl 307/311, 307/254, 307/237 [5|] Int. Cl. H03k 17/00, H03g H60[58] Field of Search 307/31], 237, 254, 307/252 L; 330/18; 250/102 [56]References Cited UNITED STATES PATENTS 3,007,06] 10/1961 Gindi 307/254Primary Examiner-John W. Huckert Assistant ExaminerB. P. DavisAttorneyJames E. Espe, Frank L. Neuhauser, Oscar B. Waddell and JosephB. Forman [57] ABSTRACT A light-emitting diode is pulsed at apreselected rate. Emitted light is conducted through a glass imageconduit to a phototransistor which operates a solid state control switchat the same rate. The solid state switch controls conduction of a seriesconnected string of transistors whose collectors swing from essentiallyzero voltage when the transistors are conducting to a high voltage whenthey are not conducting. The circuit features means for turning all ofthe transistors off in such manner that no transistor is subjected toover-voltage. The voltage pulses developed on the collector of the lasttransistor in the series may, for example, be applied to the grid of anx-ray tube to control its conduction intervals during high ratecinefluorography with an x-ray image converter. A signal that depends onimage brightness is used to modulate the width of the pulses and, hence,the output of the x-ray tube so that each film frame has uniformexposure.

4 Claims, 2 Drawing Figures TRANSISTORTZED GRlD PULSKNG CIRUUTT l OlltX-RAY TUBES AND OTHER Pllhhhii This application is a division ofcopending application, Ser. No. 801,376, filed Feb. 24, 1969 US. Pat.No. 3,567,940.

BACKGROUND OF THE lNVENTlON Cinefluorography involves converting adiagnostic x-ray image to a small bright visible image which appears onthe output phosphor of an x-ray image converter tube. A cine cameraviews the phosphor and photographs the image at a high frame rate. Theneed for a high rate is exemplified in coronary arteriography where anx-ray opaque dye is injected in the blood vessels and it is desired torecord the progress of its flow in the vessels and heart chambers. insuch cases, the x-ray tube must be turned on when a film frame isstopped and turned off when the film is being transported.

During cinerecording, the x-ray tube is turned on and off by pulsing itsgrid with a high voltage. Negative d voltage as high as 5,000 volts maybe required to cutoff an x-ray tube which has a large focal spot and 150kilovolts applied between its anode and cathode. in some cases, gridvoltage is controlled by generating a bucking voltage on the secondaryof a pulse transformer which is in series with a line from the biasvoltage source. Since a pulse transformer cannot pass a (M: component,it must be given time to recover before it can pass another pulse.Minimum recovery time turns out to be about 4 milliseconds in practice.The camera shutter is open and closed for equal time periods. At 120frames per second the shutter is open for 4 milliseconds and closed for4 milliseconds so it follows that I20 frames per second is the fastestframe rate attainable. The duty cycle, which is the ratio of pulse timeto total frame cycle time is, therefore, limited to 50%. However,radiologists now prefer to record at much higher rates in order to stopmotion properly.

Another disadvantage of using a pulse transformer is that grid voltagehas slow rise and fall times. This has required the grid pulse to beinitiated and the x-ray tube to be turned on prior to the time that thefilm frame is ready for exposure. Otherwise, the X-ll'fldlatiOnintensity may not reach its peak soon enough and a frame will beunderexposed. Premature turn-on and late turn-off of the x-ray tuberesults in the subject absorbing during each frame some radiation whichdoes not contribute toward exposing the film. This wasted radiationloads the x-ray tube unduly and adds to the cumulative dose of thesubject.

It has been proposed to use a series of silicon controlled rectiiiersfor switching the biasing source since a more nearly square biasingpulse can be attained in this way. However, several strings eachincluding a large number of SCRs in series are required to switch highvoltages. One string is needed to cause a voltage rise on a loadresistor. Another string may be'needed to supply the capacitive currentto the x-ray tube cables. Two additional strings of SCRs must beprovided for applying a blocking voltage to the other strings of SCRs sothey turn off substantially at the same time. Such systems require manycomponents and they are bulky and expensive to build. The minimum pulsewidth obtainable with SCR switching circuits is defi nitely limited asis well-known.

it has also been proposed to switch the high grid biasing voltage with aseries connected string of transistors which would permit sharp rise andfall times. Heretofore, this has not been feasible because of thedifficulty of getting all series connected transistors to turn-off andshare the supply voltage equally. As is well-known, the transistors thatremain conductive have little voltage drop across them and those thatturn off have a high voltage across them which causes avalanching andthe system may go into oscillation.

SUMMARY OF THE INVENTION An object of the present invention is toovercome the abovenoted problems by providing a series transistorswitching system that not only permits applying fast rise and fall timebiasing pulses to the grid of an x-ray tube or other device, but alsofacilitates modulating the width of the biasing pulses so that the totalx-ray intensity delivered during each pulse can be precisely controlled.This means that image brightness and film exposure can be held constantalthough the image converter is scanned over regions of varying x-rayopacity during cinefluorography and although a dye of increasing opacityis entering the field of view.

A further object is to provide a high voltage switching system thatpermits any desired duty cycle and that pertnits frame rates and pulserates that are higher than were heretofore obtainable.

A further object is to provide a fast high voltage transistor switchwhich has general application in the x-ray field such as for turning anaway tube on and off at a precise time in connection with phototimingand for turning an a-ray tube on and sustaining conduction for a longperiod of time and then turning it off such as in spot filming andfluorography.

Another object is to provide a high voltage transistor switch that hasmany uses such as for pulse modulating magnetron tubes in radar systemsas well as in x-ray systems.

A basic object is to disclose a circuit that permits taking advantage ofthe fast switching characteristics of transistors for switching highvoltages by series connecting transistors and providing means to assurethat all transistors will turn on together and that they will turn offin quick succession and share the supply voltage equally.

Still another object is to attain better electrical isolation between ahigh voltage pulse circuit and a control circuit.

Briefly, these and other objects are achieved by employingseveral'stages of series-connected high voltage transistors and acontrol transistor switching stage that controls the aforementionedstages. The conn'ol switch is triggered by the periodic flow of currentthrough a phototransistor. The phototransistor is optically coupled to alight-emitting diode with a light-conducting glass conduit or rod whichhas high dielectric strength and provides good electrical isolation. Apulse generator that is both pulse width and pulse rate modulated drivesthe diode at the chosen cine frame rate. A sensor continuously monitorsaverage brightness of the output image of an x-ray image converter and,by means of an error signal in a closed loop system, the x-ray tubeintensity is controlled at a preset level by continually modulatingpulse width.

The transistor switching circuit features means for reverse biasing thebase-emitter junctions of the transistors and thereby diverting the basestorage current of the series connected transistors so they turn off inquick succession. Reverse bias increases the hold off voltage of eachtransistor by a couple of hundred volts and contributes to short turnoff time.

A more detailed description of an illustrated embodiment of theinvention will now be set forth in reference to the drawing.

DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of a diagnosticx-ray system incorporating the invention; and,

FIG. 2 is a schematic diagram of the new transistor system for switchinghigh voltages.

DESCRIPTION OF A PREFERRED EMBODIMENT One use for the new grid pulsingsystemis in a cinefluorography system such as is depicted in FIG. 1.Here, an x-ray tube 1 projects its beam through a table top 2 and apatient 3 who is supported on the table top. The differentiallyattenuated x-rays enter an image tube 4 where they are converted to abright optical image which appears on a fluorescent screen constitutingan output phosphor 5. Electric power for operating the image tube isderived from a power supply 6.

Part of the light from the image on phosphor 5 may be deflected with adichroic mirror 8 into a tv camera 9. The image may be visualized on atv monitor 10. A spot film camera, not shown, may be substituted for thecinecamera to photograph the phosphor in a larger film size and inslower but precisely timed exposure intervals.

Part of the image light is also transmitted through dichroic mirror 8 toa cinecamera 7 which records at rates ranging from 1 to 240 or moreframes per second in a practical embodiment. Faster cameras may be used,but persistence of the image tube phosphor is ordinarily the frame ratelimiting factor in systems that use the new high speed switching systemdescribed herein. The operator sets the frame rate with a camera control11 which also operates an electric signal source comprising a pulsegenerator 12 synchronously with and at the same rate as the camera 7.The width of the output pulses from generator 12 is controlled by abrightness regulator comprising an integrator and reference l4.Brightness control involves monitoring the average brightness of theimage on output phosphor 5 with a photomultiplier tube (PM) 13. Theoutput current from PM13 is integrated and compared with a referencethat is set in accordance with the image brightness desired. The outputerror signal from integrator 14 is not merely the difference between theintegrated voltage value and the reference voltage. Rather, it is theerror signal divided by the reference signal expressed as a percentageof pulse width. This error signal is used to continually modulate thewidth of the pulses from generator 12 which results in control of theoutput image brightness.

Width modulated pulses at the selected cine recording frame rate drive alight-emitting diode (LED) 15. Light pulses from LEDIS are opticallycoupled to a phototransistor (PT) 17 through the agency of a glass lightconduit called rod 16 for brevity. Coupling a driving circuit to thedriven circuit, which is about to be described, with the light rod 16results in superior electrical isolation between the two circuits. Pulsetransformers that couple the circuits must be very highly insulatedwhich contributes toward their large size and high price.

Phototransistor 17 operates a solid state control switch 18 which ispart of the new transistor grid voltage control switch 19. When a lightpulse occurs, solid state switch 19 effectively short-circuits the gridvoltage supply so that the potential between grid 20 and cathode 21 ofthe x-ray tube is near zero in which case the x-ray tube conducts. Inthe absence of light pulses, solid state switch 19 becomes nonconductiveand removes the short circuit so that the full negative bias voltage isapplied to x-ray tube grid 20 in which case the x-ray tube stopsconducting and no output image is formed on phosphor 5.

The x-ray tube power supply 23 is essentially conventional. For presentpurposes, a three-phase supply with low ripple is preferable. The highvoltage that is applied between anode 22 and cathode 21 of the x-raytube may be set with an x-ray control 24. The setting of this voltagedepends on the penetrating power required. Control 24 also establishesthe heating current and, hence, the electron emission of the filament orcathode 21. X-ray intensity depends on the temperature of the filament.Stepless control of filament current is not required in the presentsystem. There may be one low current step for use during fiuoroscopywhen the tube is operated for long periods of time at low current. Twoadditional steps are provided for the cinefluorographic mode when thefilament is preferably brought to a higher temperature so that moreintense radiation will be available. As explained earlier, the degree offilm exposure during each frame is controlled by modulating the x-raypulse width rather than by attempting to change the intensity of thebeam by changing filament temperature during or between successive filmframes.

FIG. 2 shows the details of the new transistor circuit for switching thegrid bias voltage on x-ray tube 1. The circuit comprises a source 25 ofd-c bias voltage. The d-c source 25 may furnish voltages ranging up to5,000 volts when it is desired to control the grid of an x-ray tubewhich has a voltage of around kilovolts applied between its anode andcathode and a large focal spot size. The positive terminal of d-c source25 connects to the top of a load resistor R1. The bottom of R1 isconnected to cathode 21 of the x-ray tube 1 through a resistor R17. Thebottom of R1 is also connected to several series connected transistorstages below in which the transistors of each stage are connected in theDarlington configuration in this example. The lowermost of thetransistor stages connects to the negative side of d-c source 25. It isevident, therefore, that when the transistor stages are nonconducting,the lowermost end of R1 is relatively positive in this example. When thetransistor stages conduct, the lower end of R1 is at essentially thesame potential as the negative side of the d-c source 25. The grid 20 ofx-ray tube 1 is connected directly to the negative side of the source.Hence, when the transistors are conducting and the lower end of R1 isessentially at the same potential as the negative side of the source 25,there is no substantial potential difference between grid 20 and cathode21 and the x-ray tube 1 conducts. When the transistor stages are notconducting, a grid 20 is negative with respect to cathode 21 and thex-ray tube does not conduct.

As state earlier, the high voltage transistor switching circuit shown inFIG. 2 is turned on and off with a phototransistor 17 that receiveslight pulses at a controlled rate and width from a light emitting diode15. Phototransistor 17 conducts when it is exposed to light. As shown,phototransistor 17 controls a fast electronic switch 26 which is part ofthe control switch 11% that is encompassed in the dashed line rectangle.This switch 26 may take many known forms so it is shown in block form.Switch 26 is connected to the base of a transistor Q1. Q] in thiscircuit is conducting when phototransistor 17 is not exposed to light.When the phototransistor receives a light pulse, switch 26 effectivelyconnects the base of transistor 01 to its emitter or the negative sideof the source 25 through a connection 27. This removes the forward biasfrom the base and emitter of transistor Q1 and causes its collector togo positive. The collector resistor R28 of Q1 connects through switch 26to the positive side of the capacitor C29 which has large capacity andan essentially constant voltage on it by virtue of it being in parallelwith a zener diode 30. Capacitor C29 serves as a regulated source ofvoltage for the collector of Q1. When there is no.

lightpulse, Q1 is conducting, its collector is at essentially thepotential of its emitter and the base-to-emitter circuit of a pair ofDarlington connected transistors Q2 and O3 is not forward biased, sotransistors Q2 and 03 are nonconducting. When a light pulse occurs, Q11

stops conducting, its collector goes positive and both transistors Q2and Q3 become forward-biased. Forward-bias current is derived fromcapacitor C29 and is conducted through resistor R28, the base-emitterjunctions of Q2 and Q3 and to the negative side of the source throughforward-biased diodes D1 and D2. When Q2 and Q3 are forward biased, theybegin conducting in the usual manner from their collectors to theiremitters. This drops the potential on the emitters of Q4 and Q5 in thenext stage so that they become forward-biased by reason of the positivevoltage at point 32 of a resistive voltage divider R2-R4 driving currentthrough base resistor R5 and the bases of Q4 and Q5 and to the negativeline through Q2 and 03 which are then conducting. All other transistorstages turn on in the same manner within a couple of microseconds. Thebottom of load resistor R1 and cathode 21 of the x-ray tube are then atthe same potential as grid and the x-ray tube emits X-rays for theduration of the light pulse. From the descriptions thus far, it shouldbe evident that electronic switch 26 and transistors 01, Q2 and Q3together with their associated circuitry act as a control switch stage18 for turning the other transistors on and off in response to lightpulses or other signals.

The number of additional Darlington switching stages such as Q4, Q5, Q6and O7 is dictated by the x-ray tube grid bias voltage that is to beswitched and the hold off voltage of the individual transistors. In apractical embodiment there are eleven such Darlington stages forswitching up to 5,000 volts. Each transistor has a rated hold offvoltage of 450 volts. The Darlington configuration is used to obtain thedesired current carrying capacity.

The resistor voltage divider network comprising series connectedresistors R2, R3 and Rd has as many resistive steps as there aretransistor stages such as the one comprising Q4 and Q5. The use ofadditional resistive steps and stages is suggested by the dashed lines31 between the second and third stages counting from the bottom in FIG.2. The voltage of d-c source divides equally across these resistors. Thevoltage at a point such as 32 on the top of R2 supplies forward-biasingpotential for the base-to-emitter circuits of transistors 04 and Q5mentioned above. Bias current to these transistors is limited by anappropriate resistor such as R5. Since the total voltage at a point suchas 33 in the upper stage is higher than the voltage at point such as 32,it is necessary to provide base resistors of higher value for the upperstages. Thus, R6 and R7, the base resistors of Q6 and Q7, have a highervalue than R5. The object in any case is to bias the base circuits ofall transistors with equal current. Since the various transistor stagesare connected in series and across the d-c source 25, the voltage alsodivides equally from the collectors in one stage to the collectors ofthe transistors in adjacent stages with each stage acting as anemitterfollower. Equal potential gradient across the transistors in thevarious stages is assured by another voltage divider comprisingresistors R8, R9, R10 and R11 which compensate for collector lealragecurrent. When the first stage comprising transistors 02 and O3 is turnedon, all the other stages go on and the voltage between the respectivecollectors and emitters approaches zero. Whe the first stage is turnedoff, all other stages go off in quick succession according to theinvention and the divided voltage value appears between the respectivecollectors and emitters.

When a light pulse occurs, transistors 02 and Q3 in the first stage orcontrol switch lb turn on as has been explained above. When thishappens, the emitters of transistors Qd and Q5 in the second stage areeffectively connected to the negative side of the line as explained.This results in their base circuits being forward-biased from potentialderived at point 32. This forward-bias current through transistors Q4and Q5 renders their collector-to-emitter paths conductive. These pathshave low impedance even though they include the impedances of Q2 and Q3.When the second stage turns on it, of course, connects the emitters ofthe third stage transistors Q6 and Q7 to the negative side of the d-csource 25 and permits their bases to be forward-biased. This turn-onprocess is repeated in quick succession for as many stages as there arein the switching circuit.

There is no great difficulty in getting series connected transistors toturn on essentially simultaneously, When one stage turns on, it merelyimposes a higher potential across the other stages and the other stagesbecome conducting. Thus, the collector-to-emitter path of thetransistors will not breakdown due to overvoltage. However, transistorsare seldom connected in series to increase voltage capability because ofthe difficulty involved in forcing the voltage across each transistor tobe equal when the transistors are being turned off. When the first stagetends to turn off, its collector voltage tends to rise toward supplyvoltage which is far in excess of what the transistor is capable ofsustaining. As is known, even though the base-to-emitter bias is removedfrom a transistor, it remains on until the base storage charge isremoved.

In the new circuit, a path is provided for diverting the base storagecurrent from each transistor when the transistor is being turned off.This diverting path includes a diode such as D3 which is connectedbetween the emitter and base of Q5 so that when the diode conductsdiverted current, the base-emitter junction is reverse-biased and turnoff of the transistor is assisted. it is known that a transistor has acapability for withstanding the highest collector voltage when itsbase-toemitter circuit is reverse-biased. In the circuit underdiscussion, collector current from any stage that is still on isconducted through diodes to a capacitor. This causes reverse bias andprevents collector current from flowing through lower stages duringturn-off.

Assume that in the absence of a light pulse Q2 and Q3 tend to turn off.This will cause the voltage on the emitters of transistors Q4 and Q andon the collectors of Q2 and Q3 to rise. The rising voltage will tend tobreakdown transistors Q2 and Q3. According to the invention, however,the voltage rise on the collector of Q3 and emitter of OS that tends todevelop during turn off, causes a pair of diodes D3 and D4 to beforwardbiased in which case, current flows from the emitters of Q4 andQ5 serially through diodes D3 and D4 and another diode D5 to a capacitorC 1. Capacitor C1 is connected in series as a divider with additionalcapacitors such as C2 and C3 and the series group is connected alongwith capacitor C29 across the d-c source. The normal voltage acrosscapacitors C1 to C3 is the same as the divided voltage across thecollectors and emitters of the transistors in the individual stages.These capacitors are isolated from the base circuit of the transistorsby a reverse-biased diode such as D5. Thus, when diodes D3 and D4 areforward-biased, so is diode D5, in which case current flows from theemitters of Q4 and Q5 to capacitor C1. The forward-bias current throughdiodes D3 and D4 results in a voltage drop of about sixtenths of a voltacross each of these diodes. The direction of the voltage drop iseffective to reverse-bias the base-to-emitter junctions of Q4 and O5 inwhich case the transistors not only turn off abruptly, but are put in areverse-biased state and they are able to withstand the highestsustaining voltage.

The same turn off process occurs consecutively in the other transistorstages. For instance, when the voltage on the collectors of Q4 and 05tends to rise, so does the voltage on the emitters of Q6 and Q7 anddiodes D6 and D7 become forward-biased. This amounts to the bases of Q6and Q7 being reverse-biased by reason of current flowing through D6, D7and D8 to capacitor C2. Hence, it is seen that a voltage rise on thecollectors of any stage will be prevented by forward biasing the diodeswhich shunt the transistors in any adjacent stage.

Capacitors C 1, C2, and C3 each have a charge equalizing resistor suchas R12, R13 and R14 connected respectively in parallel with them. The RCcombinations are chosen so that the average potential on the capacitorsremains essentially the same when they receive a rapid succession ofdiverted current pulses. The resistor is of such size that its averagecurrent settles on the average current coming into the capacitor. Inother words, the average collector current times the resistance valueequals the increase of voltage on the capacitor. In a practicalembodiment, a seven volt rise on the capacitors occurred when there wasa long series of switching cycles.

Transistors Q2 and Q3 in the first or control switching stage 18 arealso reverse-biased during turn off. This may be seen by consideringthat when the light pulse disappears, Q1 switches to a conductive stateinstantly. At this time, the emitters of Q2 and Q3 are just slightlymore positive than the negative line because of the total of about 1.2volts drop across series connected diodes D1 and D2 resulting from thetransistorcollector current flowing. In parallel with diodes D1 and D2,are a pair of series connected diodes D9 and D10. The two sets of seriesconnected diodes have their cathodes connected together throughtransistor 01, when it becomes conducting, so collector current flowsfrom the emitters of Q2 and Q3 through the respective diodes D10 and D9and D1 and D2 to the negative side of the source through Q1. Currentflow through diodes D9 and D10 imposes a reverse-bias voltage on Q2 andQ3 and cuts these transistors off abruptly.

It is known that transistors such as 02 have a very small leakagecurrent between their collectors and bases to the emitters when they aresubjected to high voltage. Because of the sensitivity of the instanthigh voltage transistor circuit, it is desirable not to have any leakagecurrent flowing through the base-emitter junction or the transistors mayturn on accidentally. To preclude this, the base and emitter junctionsof the transistors are shunted by resistors such as R15 and R16. Thiscauses leakage current to take the path from the collector to the baseand through the shunting resistors without passing through the emitters.The transistors in the other stages have similar shunting resistors suchas R18, R19 and R21, R22.

In this embodiment, there is a surge current diode D11 connected betweengrid 20 of x-ray tube 1 and its cathode 21. The purpose of this is toprovide a direct circulating path in the event there is a high voltagebreakdown between the anode and grid of the x-ray tube 1 so that faultcurrent will not enter the switching circuit. Diode D1 1 must be capableof withstanding the full grid biasing voltage. Another diode D12 isconnected in parallel with load resistor R1 to protect it against theheavy reverse currents that would flow if there were a breakdown in thex-ray tube. Resistor R17 which is connected between the load resistor R1and the x-ray tube cathode 21 is for limiting the surge current thatflows when the series transistors turn on. The surge current resultsfrom capacitance of the x-ray tube cables which charge when the biasvoltage is applied to the x-ray tube and discharge when the seriesconnected transistors become conductive.

In FIG. 2 there is shown a conductor 34 between the midpoint of the d-csource voltage and the midpoint of the resistor voltage divider networkR2-R4. This conductor provides a path for some of the biasing currentthat flows to the bases of the various transistors through resistorssuch as R5. Otherwise, other resistors in the network such as R4 wouldhave to carry the sum of the biasing currents for all of the transistorsand resistor R4 would have to have a high wattage rating.

Commercial phototransistors 17 are provided with an external baseterminal so that the base-to-emitter junction can be forward-biased ifdesired to thereby increase the sensitivity of the phototransistor.External biasing is omitted in this case because it was found that thephototransistor becomes extremely sensitive to noise if there isexternal biasing.

The new transistorized switching circuit of FIG. 2 has demonstrated acapability of permitting einerecording at rates as high as 500 framesper second, if desired, which is well above the highest frame rate ofpresently available cameras. The pulse rate may be in the kilocyclerange if the application requires. The new circuit is also distinguishedby switching so fast that it permits effective radiation pulse widthsranging down to zero if desired. The triggering light pulses and thecorresponding x-ray pulses are relatively square. No excess radiation isadministered to the patient by turning on the x-ray tube prematurely aswas required heretofore in order to assure that its output will peakbefore a frame is exposed. Constant film density is obtained bydetecting the average brightness of the output phosphor in the imageconverter tube and using the signal so developed to modulate the widthof the x-ray pulses. This results in better exposure control than isobtainable by attempting to control x-ray tube filament current orapplied voltage.

Although construction and use of the new transistor switching circuit isillustrated in connection with a grid controlled x-ray tube, thoseversed in the electrical arts will appreciate that the new circuit maybe used in many other high voltage switching applications which have notheretofore been possible because of the difficulty of compelling seriesconnected transistors to share source voltage equally during turn off.

I claim:

1. A high voltage switching circuit comprising:

a. a d-c voltage source having first and second output terminals ofopposite polarity,

b. a load resistor having one end connected to said first sourceterminal,

c. transistor means having a base, an emitter and a collector the latterof which is connected to the other end of said load resistos,

d. a resistor voltage divider connected between said first and secondsource terminals,

e. a'base resistor connected between an intermediate point of theresistor divider and said transistor base,

f. a control switch connected between the emitter of said transistor andsaid second source terminal, said switch being selectively operable toconduct forward-bias current through the base-to-emitter circuit of saidtransistor to turn it on and to interrupt said bias current to turn itoff,

g. a capacitor voltage divider connected between said first and secondsource terminals,

h. a first diode means connected between an intermediate point of thecapacitor voltage divider and the base of said'transistor means,

. a second diode connected between the emitter and base of saidtransistor and being biased opposite of the forward-bias direction ofthe base-to-emitter path of the transistor and being in series with thefirst diode means, whereby when said control switch is nonconductive theresulting voltage rise on said transistor emitter will cause currentflow to said capacitor and forward-bias said second diode andreverse-bias the emitter and base of the transis' tor to turn it offthereby causing a voltage rise between the collector and the secondsource tenninal.

2. The invention set forth in claim 1 including:

a. a phototransistor controlling conduction of said control switch;

b. a light source optically coupled transistor;

c. an electric signal source adapted to energize said light source fortimed intervals to thereby turn said with said photocontrol switch onand off.

3. The invention set forth in claim 2 wherein:

a. said light source is a light-emitting diode and a light conduitcouples the same to said phototransistor.

4. A transistor switching system for controlling the bias voltagebetween the grid and cathode of an x-ray tube and the like, comprising:

a. a d-c bias voltage source having positive and negative terminals,

b. a resistor voltage divider connected across said source terminals,

c. a load resistor having one end connected to the positive sourceterminal,

d. a plurality of series connected transistor stages each of which has abase, an emitter and a collector, the collector of one being connectedto the other end of the load resistor and the emitter of the one beingconnected to the series connected collector-emitter paths of the others,

e. a control switch connected between the last emitter in the series andthe negative source terminal,

f. base resistors connected between successive intermediate points ontheresistordivider and the re spective transistor bases whereby tosupply forward-bias current through the base-emitter junctions of thetransistors when said control switch is conducting to turn the seriesconnected transistors on and thereby make the potential of the other endof the load resistor substantially equal to that of the negative sourceterminal,

g. a voltage divider comprising series connected capacitors that areconnected across said source terminals,

h. a first diode connected between each transistor base and acorresponding intermediate point on the capacitor voltage divider, saiddiodes being biased oppositely of the base-emitter junctions,

. a second diode connected in parallel with each base-emitter junctionand biased in the direction of the first diode and being in series withit to provide a conductive path between each emitter and its associatedcapacitor,

j. the turning off of said control switch causing successive increasedpotentials on the transistoremitters, beginning with the stage next tothe control switch, which causes collector-emitter current from thestage to divert through the second and first diodes into the saidcapacitor whereby to reverse bias the base-emitter junctions of theseries connected transistors in succession during turn off,

. a discharge resistor in parallel with each capacitor,

and

. a collector leakage current resistor shunting the emitter andcollector of path of each transistor, said leakage current resistorsbeing series connected to each other and connected between said sourceterminals.

m. turn off of said series transistor stages causing the voltage to risepositively on said other end of the load resistor with respect to saidsecond source terminal.

1. A high voltage switching circuit comprising: a. a d-c voltage sourcehaving first and second output terminals of opposite polarity, b. a loadresistor having one end connected to said first source terminal, c.transistor means having a base, an emitter and a collector the latter ofwhich is connected to the other end of said load resistor, d. a resistorvoltage divider connected between said first and second sourceterminals, e. a base resistor connected between an intermediate point ofthe resistor divider and said transistor base, f. a control switchconnected between the emitter of said transistor and said second sourceterminal, said switch being selectively operable to conduct forward-biascurrent through the base-to-emitter circuit of said transistor to turnit on and to interrupt said bias current to turn it off, g. a capacitorvoltage divider connected between said first and second sourceterminals, h. a first diode means connected between an intermediatepoint of the capacitor voltage divider and the base of said transistormeans, i. a second diode connected between the emitter and base of saidtransistor and being biased opposite Of the forward-bias direction ofthe base-to-emitter path of the transistor and being in series with thefirst diode means, whereby when said control switch is nonconductive theresulting voltage rise on said transistor emitter will cause currentflow to said capacitor and forward-bias said second diode andreverse-bias the emitter and base of the transistor to turn it offthereby causing a voltage rise between the collector and the secondsource terminal.
 2. The invention set forth in claim 1 including: a. aphototransistor controlling conduction of said control switch; b. alight source optically coupled with said phototransistor; c. an electricsignal source adapted to energize said light source for timed intervalsto thereby turn said control switch on and off.
 3. The invention setforth in claim 2 wherein: a. said light source is a light-emitting diodeand a light conduit couples the same to said phototransistor.
 4. Atransistor switching system for controlling the bias voltage between thegrid and cathode of an x-ray tube and the like, comprising: a. a d-cbias voltage source having positive and negative terminals, b. aresistor voltage divider connected across said source terminals, c. aload resistor having one end connected to the positive source terminal,d. a plurality of series connected transistor stages each of which has abase, an emitter and a collector, the collector of one being connectedto the other end of the load resistor and the emitter of the one beingconnected to the series connected collector-emitter paths of the others,e. a control switch connected between the last emitter in the series andthe negative source terminal, f. base resistors connected betweensuccessive intermediate points on the resistor divider and therespective transistor bases whereby to supply forward-bias currentthrough the base-emitter junctions of the transistors when said controlswitch is conducting to turn the series connected transistors on andthereby make the potential of the other end of the load resistorsubstantially equal to that of the negative source terminal, g. avoltage divider comprising series connected capacitors that areconnected across said source terminals, h. a first diode connectedbetween each transistor base and a corresponding intermediate point onthe capacitor voltage divider, said diodes being biased oppositely ofthe base-emitter junctions, i. a second diode connected in parallel witheach base-emitter junction and biased in the direction of the firstdiode and being in series with it to provide a conductive path betweeneach emitter and its associated capacitor, j. the turning off of saidcontrol switch causing successive increased potentials on thetransistor-emitters, beginning with the stage next to the controlswitch, which causes collector-emitter current from the stage to divertthrough the second and first diodes into the said capacitor whereby toreverse bias the base-emitter junctions of the series connectedtransistors in succession during turn off, k. a discharge resistor inparallel with each capacitor, and l. a collector leakage currentresistor shunting the emitter and collector of path of each transistor,said leakage current resistors being series connected to each other andconnected between said source terminals. m. turn off of said seriestransistor stages causing the voltage to rise positively on said otherend of the load resistor with respect to said second source terminal.